Understanding the structure-function relationships of hemoglobin (Hb) is of great biomedical significance. The two-state allosteric model of Monod, Wyman, and Changeux (MWC) has long been used to provide a description of a wide variety of structural, equilibrium and kinetic data on cooperative oxygen binding to Hb. Recent work performed under the above-mentioned parent grant has challenged the fundamental assumption underlying the MWC model and stating that oxygen affinity is primarily determined by the quaternary structure. Work performed in my laboratory shows that an extended MWC allosteric model is required to fully account for the variation of oxygen binding isotherms induced by allosteric effectors. This proposal aims at investigating features of the extended MWC model suggested by our ligand binding studies using a combined experimental and computational approach designed to complement our previous work using HbA hybrids. It features a high-resolution FLN laser spectroscopy experimental approach which will be used - not only for the acquisition of vibronically-resolved spectra - but also to perform pressure studies. The essential originality of this proposal resides in the application of a spectroscopic technique designed to monitor hydrostatic pressure as a thermodynamic parameter combined to a computational approach aimed at modeling the effect of binding allosteric effectors on the Hb tertiary and quaternary structure. The Hungarian laboratory (Fidy) will carry out the high-resolution spectroscopy experiments and pressure studies as well as the computational work. The synthesis of the required HbA hybrids will be done in my laboratory (Yonetani) as well as the FT-Raman experiments.